Multilayer mirrors are widely used as X-ray monochromators in synchrotron radiation facilities. Compared with crystal monochromators, multilayers have variable period thicknesses and can be applied at different energies. At the same time, the energy bandwidth of the multilayers is 1-2 orders of magnitude larger than that of the crystals, which can provide higher photon flux. As mirrors in synchrotron radiation beamlines operate under grazing incidence conditions, larger mirrors are usually required to fully receive the beam. In addition, a double-channel multilayer composed of two different structural material pairs is usually deposited on the surface of the mirror to make the beamline cover a wider energy range. In recent years, China's synchrotron radiation facilities have been continuously upgraded and built, including the Shanghai Synchrotron Radiation Facility (SSRF) and Beijing High Energy Photon Source (HEPS). In some beamlines, single-channel multilayer mirrors are no longer sufficient, and double-channel multilayer mirrors are required. Driven by these applications, a large-size double-channel multilayer mirror is developed in this paper.
The double-channel multilayers used a combination of W/Si and Ru/C multilayers, and Ru/C multilayers and W/Si multilayers work in the energy range of 10-18 keV and 18-25 keV, respectively. The W/Si and Ru/C multilayer samples are fabricated in a linear magnetron sputtering system. The base pressure before the deposition is 9.5×10-5 Pa and the working gas uses high-purity argon (volume fraction of 99.999%). A series of experiments are first carried out on Si wafers mainly to optimize the quality and thickness uniformity of the multilayers. The uniformity in the length direction can be ensured as long as the stability of the motion rate is guaranteed, and the uniformity in the width direction can be controlled by installing a crescent-shaped mask in front of the target. Then, W/Si and Ru/C double-channel multilayers are deposited on the surface of a high-precision Si plane mirror based on the optimized results. The areas of the two multilayer stripes are both 320 mm×20 mm, and the interval is less than 3 nm. After deposition, the multilayer samples are characterized by grazing incidence X-ray reflectometry (GIXR) at 8.04 keV using an X-ray diffractometer. The GIXR curve is fitted by IMD software to obtain thickness, density, and interface width. The non-specular scattering tests of the multilayers are also conducted on an X-ray diffractometer. The surface morphologies of the multilayers are measured by atomic force microscopy (AFM) and then one-dimensional power spectrum density (PSD) functions are calculated.
A W/Si and Ru/C double-channel multilayer mirror is fabricated in this paper. After process optimization, within the range of 320 mm length and 20 mm width, the RMS error of the thickness of the W/Si multilayer is 0.30% and 0.19%, and that of the Ru/C multilayers is 0.39% and 0.20%, which has almost reached the world-class level. Finally, on the basis of the optimized experimental results, W/Si and Ru/C multilayers are deposited on a high-precision Si plane mirror with a size of 350 mm×60 mm in two stripes, and the estimated reflectivity (8.04 keV) is 68% and 65%, respectively. The multilayer mirrors can meet the requirements of the beamline and are successfully applied in the membrane protein beamline of SSRF. In future research, uniformity can be improved by increasing mask fabrication, mounting accuracy, and substrate movement rate stability.